The tremendous need for bridge replacements in the nation has caused a renewed interest in timber bridges. The advantages of modern timber bridges have been described in detail, see Brungraber, R., Gutkowski, R., Kindya, W., and McWilliams, R. in "Timber Bridges: Part of the Solution for Rural America," Transportation Research Record, No. 1106, 1986, pp. 131-139 and Verna, J. R., Graham, J. F., Shannon, J. M., and Sanders, P. H. in "Timber Bridges: Benefits and Costs," Journal of Structural Engineering, ASCE, July 1984, pp. 1563-1571. Most importantly, timber bridges are economical and have a long life expectancy. Because timber bridges are easy to construct, a large percentage of the savings is due to reduced construction cost. Unlike steel and concrete, timber is resistant to deicing salts and is generally expected to have a 50 year life span. Furthermore, timber bridges can be built year round, are lightweight which often allows the us of existing substructures, and are often more aesthetic than other low cost alternatives.
Stress-laminated timber bridges are a relatively new type of timber bridge design that have a strong potential for increased use. A stress-laminated bridge deck consists of on-edge longitudinal timbers post-tensioned transversely with high strength steel rods. The rods run across the width of the bridge and are located inside the deck in pre-drilled holes in the laminate. The design was developed about ten years ago in Ontario by Taylor and Csagoly as a means of rehabilitating simple nail laminated timber bridges, see Taylor, R. J., and Csagoly, P. F., "Transverse Post-Tensioning of Longitudinally Laminated Timber Bridge Decks," Transportation Research Record, No. 665, 1978, pp 236-244 and Csagoly, P. F. and Taylor, R. J., "A Structural Wood System for Highway Bridges," Structural Research Report Srr-80-05, Ontario Ministry of Transportation and Communications, 1980.
A series of laboratory tests were conducted by Taylor et al were reported in Taylor, R. J., DeV. Batchelor, B., and Van Dalen, K., "Prestressed Wood Bridges," Structural Research Report SRR-83-01, Ontario Ministry of Transportation and Communications, 1983 and address some fundamental questions concerning behaving of stressed timber decks. For the deck to perform properly, the prestress level in the deck must cause sufficient friction between the laminates to resist wheel loads without slipping. After initial tensioning, the wood creeps under the applied compression, resulting in loss of stress in the tensioning rods. The prestress force is also effected by temperature changes and moisture content. The effects of wood species, stiffness ratio, tensioning sequence, and relative humidity on prestress loss were examined. As a result of these tests, it was found that the stress ratio, defined as the final prestress level divided by the initial, did not fall below 50%. Consequently, the value of 50% loss of prestress due to wood creep was adopted in the Ontario Highway Bridge Design Code (OHBDC) for design purposes.
Taylor and Csagoly have shown that retention of the initial prestress is improved by decreasing the stiffness ratio of the bridge, see "Transverse Post-Tensioning of Longitudinally Laminated Timber Bridge Decks", supra. The stiffness ratio is defined as the axial stiffness of the prestressing system to that of the wood. This was determined on a laboratory scale under relatively low tensile forces, i.e. about 18 kips, by using large coil springs in series with tensioning rods in a creep testing machine. Use of large coil springs is technically and economically unfeasible with respect to an actual bridge deck, i.e. coil spring assemblies capable of exterting sufficient force to provide a bridge deck having a load carrying capacity of practical significance would be prohibitively unwieldy and expensive.
What is needed in the art is a practical way to address the above discussed difficulties and provide a stress laminated bridge deck that is resistant to prestress losses.